Blood
Vessels in Man
In Man with closed circulatory systems, the blood flows within
vessels of varying sizes. All vertebrates, including humans, possess this type
of circulation.
The external structure of the heart has many blood vessels that
form a network, with other major vessels emerging from within the structure.
The blood
vessels typically comprise the following:
Veins supply deoxygenated blood to the heart via inferior and
superior vena cava, and it eventually drains into the right atrium.
Capillaries are tiny, tube-like
vessels which form a network between the arteries to veins.
Arteries are muscular-walled
tubes mainly involved in supplying oxygenated blood away from the heart to all
other parts of the body. Aorta is the largest of the arteries and it branches
off into various smaller arteries throughout the body.
There are certain important arteries and veins
which are present in heart and are as follows:
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•Inferior vena cava: It brings deoxygenated
blood from lower part of the body to the right atrium.
•Superior vena cava: It brings deoxygenated
blood from upper part of the body to the right atrium.
•Pulmonary artery: Right ventricle pumps
deoxygenated blood into pulmonary artery and this artery carries this
deoxygenated blood to the lungs for purification.
•Pulmonary vein: It carries oxygenated blood
from lungs and transfers it to left atrium.
•Aorta: It receives oxygenated blood from the
left ventricle and then transfers this blood to other arteries which circulate
that oxygenated blood to all other body parts.
A separation also remains present between left
and right portion of heart which we call as septum. This septum separates the
left and right chambers of heart and thus prevents mixing of deoxygenated and
oxygenated blood.
Arteries
An artery is
a blood vessel that conducts blood away from the heart. All arteries have
relatively thick walls that can withstand the high pressure of blood ejected
from the heart. However, those close to the heart have the thickest walls,
containing a high percentage of elastic fibers in all three of their tunics.
This type of artery is known as an elastic artery.
The tunica
intima (also called the tunica interna) is composed of
epithelial and connective tissue layers.
The tunica
media is the substantial middle layer of the vessel wall.
It is generally the thickest layer in arteries.
The outer tunic,
the tunica externa (also
called the tunica adventitia), is a substantial sheath of connective tissue
composed primarily of collagenous fibers.
Vessels
larger than 10 mm in diameter are typically elastic. Their abundant elastic
fibers allow them to expand, as blood pumped from the ventricles passes through
them, and then to recoil after the surge has passed. If artery walls were rigid
and unable to expand and recoil, their resistance to blood flow would greatly
increase and blood pressure would rise to even higher levels, which would in
turn require the heart to pump harder to increase the volume of blood expelled
by each pump (the stroke volume) and maintain adequate pressure and flow.
Artery walls would have to become even thicker in response to this increased
pressure. The elastic recoil of the vascular wall helps to maintain the
pressure gradient that drives the blood through the arterial system. An elastic
artery is also known as a conducting artery, because the large diameter of the
lumen enables it to accept a large volume of blood from the heart and conduct
it to smaller branches.
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Capillaries
A capillary is
a microscopic channel that supplies blood to the tissues themselves, a process
called perfusion. Exchange of gases and other substances occurs in
the capillaries between the blood and the surrounding cells and their tissue
fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10
micrometers; the smallest are just barely wide enough for an erythrocyte to
squeeze through. Flow through capillaries is often described as microcirculation.
The
wall of a capillary consists of the endothelial layer surrounded by a basement
membrane with occasional smooth muscle fibers. There is some variation in wall
structure: In a large capillary, several endothelial cells bordering each other
may line the lumen; in a small capillary, there may be only a single cell layer
that wraps around to contact itself.
For
capillaries to function, their walls must be leaky, allowing substances to pass
through. There are three major types of capillaries, which differ according to
their degree of “leakiness:” continuous, fenestrated, and sinusoid capillaries.
Veins
A vein is a blood vessel that conducts blood toward the heart.
Compared to arteries, veins are thin-walled vessels with large and irregular
lumens
Because they are low-pressure vessels, larger
veins are commonly equipped with valves that promote the unidirectional flow of
blood toward the heart and prevent backflow toward the capillaries caused by
the inherent low blood pressure in veins as well as the pull of gravity.
Their ability to hold this much blood is due to
their high capacitance,
that is, their capacity to distend (expand) readily to store a high volume of
blood, even at a low pressure.
This volume of blood is referred to as venous reserve.
Through venoconstriction, this “reserve” volume of blood can get back to the
heart more quickly for redistribution to other parts of the circulation.
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